The present invention relates to a device and a method providing the possibility, by means of fiberoptical systems, to generate interference-free images and relates in particular to how the picture of an imaging system, wherein an image is imaged to a sensor by means of a bundle of several orderly fibers, may operate in a suitable way to obtain an image without interfering structures.
Optical systems in which an image is transferred to an imaging sensor via optics, are widely used. Without the imaging use of endoscopes, today many applications among others in the fields of diagnostics, inspection, quality assuring and research would be unthinkable. Here, on the one hand, systems of refractive optics (lens-optical systems) are used, i.e. systems with a rigid arrangement within which the image is transmitted to the sensor by an arrangement of lenses similar to a lens/objective of a camera. On the other hand, fiberoptical systems are used which consist of a great number of orderly light-conducting fibers combined into a bundle, wherein the light is guided to a sensor through the plurality of fibers.
The current preference for lens-optical systems is among others due to the image quality. Wherever a literally more “flexible” use is needed (small, difficult access), high quality, semi-rigid or flexible endoscopes (fiberscopes) with low operating diameters and glass fiber image transmitters have to be used. With the use of such a fiberoptical system of several image transmitters, typically a finite range of the object to be observed is transmitted by each individual image conductor which is used. As with acceptable diameters of the complete fiber bundle no randomly high number of individual fibers is possible and individual fibers cannot be produced with arbitrarily low diameters, hitherto especially the bad resolution of the transmitted image data and the architecture conditioned honeycomb structure of such a system inhibits an adequate use of these devices.
The image transmitter of high quality fiberscopes consists of a regularly arranged bundle of about 5000 to 8000 individual fibers. Compared to the resolution of a conventional full-motion picture camera (e.g. VGA: 640×480>30000 image points and/or pixels), this value is thus far below the threshold value for sensible applications. Typically, the image signal transported by means of the individual fibers is observed using such a conventional full-motion picture camera. The individual optical fibers or optical waveguides, respectively, usually comprise a sheathing, so that from the sheathing interfering structure in the observed image result which may for example be smoothed by low-pass filters or be adaptively reduced by spectral masking. In order to remove the structures introduced by the honeycomb-like structure which strongly interfere the assessment of an image, there are already solutions which interpolate a honeycomb-free image on the basis of luminosity information of the fibers. Likewise, smoothing of the honeycomb-shaped sheathing structures is for example achieved by masking the same in the Fourier space. Maskings here have the disadvantage that they do improve the optical impression of the recorded image, but do not increase the accuracy by which the imaging location may be determined.
One problem which is generally to be solved is dealt with in the German Patent DE 4318140 A1. It is described here, how the positions of the light points may be determined which are imaged by the individual glass fibers to a higher resolution sensor. The Patent Specification shows, how an assignment of the location of the fibers on the input side of the light fiber bundle to the position of the light points caused by the fibers on the sensor is possible on the basis of the determined fiber coordinates. By this method, a wrong imaging is prevented as it is caused by non-parallel fibers in the fiber bundle, this way, however, no resolution increase may be achieved.
Methods of the prior art for image processing an image recorded by means of a fiberoptical system have the disadvantage here that the illustration quality and/or the subjective quality of perception of the pictures is improved, that the same do not cause an actual increase of resolution, however, as for increasing the resolution an introduction of additional (image) information is needed.
If the geometry of the observed scene and/or the occurrence of certain geometrical shapes within the scene is already known beforehand, this knowledge may he introduced for each individual picture in order to actually increase the resolution (e.g. by the application of edge-maintaining filters). If it is for example known that within the picture a straight intensity leap is present, by the suitable application of a filter the course of the edge within the picture may be determined with a higher precision than the resolution of the individual image point. In fiberoptical systems which are used for diagnostics, the object and/or the shape of the object to be observed is usually not known a priori, however, so that such methods are not generally possible.
In principle, the information difference and/or the information redundancy of several successive pictures from varying observation positions or directions may be combined in order to reconstruct an image which has a higher resolution than an individual picture. For conventional video sequences, i.e. successive individual images which consist of a rectangular grid of image points, such methods are used under the collective term “superresolution”. In addition to that, there are first approaches for extending the method for increasing the resolution for image points which are present in random grid structures, i.e. in non-rectangular coordinates.
By means of motion detection, thus in principle also for fiberscopes an effective increase of image resolution is possible. For an effective implementation of motion detection algorithms (motion compensation) it is necessitated however, to provide images as a basis for motion detection which are free from artefacts, i.e. from additional structures covering the interesting image content.
As already mentioned above, there are some approaches to make an image, which was recorded by means of a fiberscopic system, free from artefacts and/or to smooth and/or interpolate the honeycomb-shaped structures which are present in systems. Here it is indicated among others in the unpublished German Patent Application No. 10200604006.6, how for a plurality of pixels of a pixel array illuminated by means of a single optical fiber one individual intensity value each may be determined which is used as the image information transmitted by the image transmitter. Image analysis and processing algorithms based on the same are based on the fact that for each individual optical fibre only one single image point is used, so that in additional steps a honeycomb-free image may be generated by an interpolation between the individual image points of the fibre. This interpolated image serves as a basis for the subsequent motion compensation.
Here, the one intensity is used as image information which results from a summation of the pixels which are located in a predetermined radius around a central pixel. The method described there and other methods corresponding to the prior art have the big disadvantage here that the localization of the fiber centres on the sensor may only take place with pixel accuracy, i.e. with the accuracy predetermined by the size of an individual light-sensitive pixel. This deteriorates the applicability of images generated this way for a subsequent motion compensation which is based on generating additional image information by motion detection, wherein the motion detection may be performed with an accuracy which is below the pixel size. A localization of the fiber centers underlying the motion compensation with pixel accuracy thus inevitably leads to a resolution which remains far below the possible theoretical value.
In addition to that, the summation of the light-intensities in a circular range around a central pixel is only restrictedly suitable to describe the complete intensity transmitted by an optical fiber. For example, a deviation from a circular geometry of the light point as it may occur when an individual fiber and/or the fiber bundle comprises an angle relative to the sensor leads to the overall intensity not being determined correctly.